Influence of surface roughness on a spray cooling system with R134a. Part I: Heat transfer measurements

Experimental measurements in a spray cooling test rig were carried out for two different heater surface roughnesses and for two different types of nozzles with the dielectric refrigerant R134a. In this paper, results of the heat transfer measurements are presented. The analysis of the results explains the influence of the volumetric flow rate, the surface roughness and the type of nozzle (through the spray parameters) on the spray cooling boiling curve, on the Nusselt number and on the efficiency. It has been found that the effect of a smooth roughness is to delay the onset of the nucleate boiling, but once this regime has started, boiling is so fast that the CHF (Critical Heat Flux) is reached at lower heater temperatures and lower heat fluxes. In a companion paper the sprayed refrigerant film thickness measurements and its relation with the heat transfer measurements are presented.

► The use of a smooth roughness surface delay the onset of the nucleate boiling.
► But the CHF is reached at lower heater temperatures and lower heat fluxes.
► For a smooth roughness, the CHF point is independent of the nozzle type.
► At the CHF point, efficiency increases as the surface roughness increases.